In the field of lubricating oils, additives have been added to lubricating base oils such as highly refined mineral oils, to improve the properties such as the viscosity-temperature characteristics or heat and oxidation stability of the lubricating oils (for example, see Patent documents 1-9).
For example, lubricating oils used in internal combustion engines such as automobile engines require heat and oxidation stability that allows them to withstand harsh conditions for prolonged periods. In order to ensure heat and oxidation stability for conventional internal combustion engine lubricating oils, it is common to use highly refined base oils such as hydrocracked mineral oils or high performance base oils such as synthetic oils, with addition of peroxide-decomposable sulfur-containing compounds such as zinc dithiophosphate (ZDTP), molybdenum dithiocarbamate (MoDTC), or ashless antioxidants such as phenol-based or amine-based antioxidants to the base oils.
In motorcycles with 4-stroke internal combustion engines, on the other hand, lubrication of the internal combustion engine, transmission and wet clutch is accomplished using the same lubricating oil. The lubricating oil used, therefore, must exhibit properties suitable for lubrication of transmissions and wet clutches, in addition to the performance generally required as an automobile lubricating oil. For this reason it is not desirable to apply lubricating oils for four-wheeled vehicles directly as lubricating oils for such motorcycles, and therefore research is being conducted toward developing lubricating oils suitable for 4-stroke internal combustion engines for motorcycles (see Patent document 7, for example).
With the recent emphasis on environmental issues including reduction of carbon dioxide gas emissions, the goal of reducing energy consumption (fuel efficient) in automobiles, construction equipment, agricultural machinery and the like has become a matter of urgency, and it is highly desirable for drive-trains such as gearboxes and final reduction gears to help contribute to reduced energy consumption. Fuel efficiency of drive-trains can be achieved by methods that lower the viscosity of the lubricating oil to reduce stirring resistance and friction resistance against the sliding surfaces. For example, gearboxes used as automobile automatic transmissions or continuously variable transmissions comprise a torque converter, wet clutch, gear bearing mechanism, oil pump, overpressure control mechanism and the like, while manual transmissions and final reduction gears include a gear bearing mechanism, and by reducing the viscosity of the lubricating oils used therein to lower stirring resistance and friction resistance, it is possible to improve power transmission efficiency and achieve fuel savings. However, reducing the viscosity of the lubricating oil also results in lower lubricity (wear resistance, prevention of seizure properties, fatigue life, etc.), which is disadvantageous for gearboxes. Also, addition of phosphorus-based extreme-pressure agents to guarantee wear resistance for lubricating oils with reduced viscosity can significantly shorten the fatigue life. In addition, while sulfur-based extreme-pressure agents are effective for improving fatigue life, it is generally known that the effect of the lubricating base oil viscosity in low viscosity lubricating base oils is greater than that of the additives. One strategy for ensuring lubricity when lowering the viscosity of lubricating oils for increased fuel efficiency has been to optimize the combinations of phosphorus-based extreme-pressure agents and sulfur-based extreme-pressure agents added to lubricating base oils (for example, see Patent documents 8 and 9).    [Patent document 1] Japanese Unexamined Patent Publication HEI No. 4-36391    [Patent document 2] Japanese Unexamined Patent Publication SHO No. 63-223094    [Patent document 3] Japanese Unexamined Patent Publication HEI No. 8-302378    [Patent document 4] Japanese Unexamined Patent Publication HEI No. 9-003463    [Patent document 5] Japanese Unexamined Patent Publication HEI No. 4-68082    [Patent document 6] Japanese Unexamined Patent Publication HEI No. 4-120193    [Patent document 7] Japanese Unexamined Patent Publication No. 2003-41283    [Patent document 8] Japanese Unexamined Patent Publication No. 2004-262979    [Patent document 9] Japanese Unexamined Patent Publication No. 2004-262980